We propose to test the following hypothesis: 1) The pregnancy related increase in uterine blood flow mediated by release of nitric oxide (NO) is flow dependent; 2) The mechanism by which this flow-dependent alteration of the vascular tone occurs is partly mediated by release of bradykinin (Bk) by endothelial cells, which in turn releases NO; 3) The mechanism of the flow dependent alteration of the vascular tone during pregnancy is also partly dependent on the release of NO by activation of the potassium channel; 4) The increase in serum concentrations of estradiol and progesterone during pregnancy is also responsible for the flow related release of NO by the uterine vascular bed. Five specific aims are proposed to achieve our objective. Specific Aim 1 We will assess whether the decrease in tone of th uterine vascular bed during pregnancy is mediated by flow-dependent release of NO. This specific aim will test hypothesis 1. Specific Aim 2 We will assess the modulating role of endothelium derived bradykinin in the flow dependent release of NO from the perfused uterine vascular bed in vitro. This specific aim will test hypothesis 2. Specific Aim 3 We will assess the role of K+channel activation on the flow related release of NO from the perfused uterine vascular bed. This specific aim will test hypothesis 3. Specific Aim 4 We will assess the role of estradiol and progesterone in modulating tone in the uterine microvasculature and also the cellular and molecular mechanisms involved. This specific aim will test hypothesis 4. Specific Aim 5 We will assess the mechanism of increase in uterine blood flow during pregnancy utilizing an in vivo model. This specific aim will test some key parts of hypothesis 1-4 in vivo. An understanding of the basic mechanism involved in modulating uterine blood flow (UBF) during pregnancy will have important basic and clinical implications. The results of these studies will indicate the local regulatory mechanism by which No is released and how sex steroids can modify this effect. The results of these studies would lay the foundation for our understanding of how these mechanisms may be altered in certain disease states associated with pregnancy such as hypertension, diabetes, and intrauterine growth retardation conditions where the fetus may be adversely affected.